This invention relates to north-south pincushion distortion correction circuits suitable for use with television display systems.
North-south pincushion distortion is seen as a bowing inwardly of horizontal scanning lines of a raster towards the center of the raster. This distortion is caused primarily by the deflection of the scanning beams, having a given radius from their deflection center to the center of a viewing screen, over the viewing screen which has a larger radius of curvature. As the beam deflection angle of the picture tube increases, and as flatter viewing screens are utilized, the pincushion distortion increases.
It is known that north-south pincushion distortion can be corrected by modulating the vertical rate sawtooth scanning current at the horizontal rate with a properly phased parabolic wave or sine wave approximation of a parabolic wave. This correction component decreases in amplitude from the top of the raster to the center, at which point its phase is reversed, and increases in amplitude from the center to the bottom of the raster. This correction circuit superimposed on the vertical scan current produces the familiar bow tie waveform pattern.
U.S. application Ser. No. 595,809, filed July 14, 1975 issued Sept. 13, 1977 as U.S. Pat. No. 4,048,544 for Peter Eduard Haferl and entitled "SWITCHED VERTICAL DEFLECTION SYSTEM" discloses an arrangement in which horizontal rate energy is coupled to charge a capacitor in parallel with vertical deflection coils, the discharge of the capacitor through the coils providing the vertical scanning current. For the first half of the vertical interval horizontal energy of one polarity and in decreasing amounts charges the capacitor. During the second half of the vertical interval horizontal energy of the opposite polarity and in increasing amounts charges the capacitor for providing the sawtooth scanning current. The horizontal rate voltage across the capacitor is integrated by the inductance of the vertical deflection coils and produces a horizontal rate current component which modulates the vertical scanning current. U.S. application Ser. No. 618,842 issued Aug. 9, 1977 as U.S. Pat. No. 4,041,354 entitled .tbd.Pincushion Correction Circuit" discloses an arrangement for utilizing this modulation to achieve north-south pincushion distortion correction. Specifically, the latter-mentioned application discloses an arrangement including two capacitors and a resonant circuit coupled to the charging capacitor for phase shifting the horizontal rate energy prior to injecting this current between the two series connected vertical deflection coils.
The phase of the horizontal rate pincushion distortion correction current component must be carefully controlled for maximum effectiveness. A manufacturer may decide to utilize a relatively high impedance deflection yoke such as one utilizing toroidal vertical windings and saddle-type horizontal windings in which the inductance of the vertical windings, even in parallel, is in the order of 20 mh compared to an inductance of about 2 mh for series-connected vertical windings of a low impedance precision type full toroidal yoke. The combination of the winding inductance and the parallel capacitance described above must be selected for determining the vertical retrace period since these elements form a resonant circuit which reverses the polarity of vertical scanning current during the retrace interval. Thus, a higher impedance deflection winding would require a smaller parallel capacitor to maintain the same retrace period. Such a smaller capacitance would present a higher impedance to the horizontal rate charging current and would phase shift the horizontal rate voltage across the capacitor. In order to provide the proper phase for north-south pincushion distortion correction some arrangement for shifting the phase must be provided. As a practical matter such a circuit must also be cost effective and not disrupt the operation of the vertical deflection current generator.